A circular dependency occurs when two classes depend on each other. For example, class A needs class B, and class B also needs class A. Circular dependencies can arise in Nest between modules and between providers.
While circular dependencies should be avoided where possible, you can't always do so. In such cases, Nest enables resolving circular dependencies between providers in two ways. In this chapter, we describe using forward referencing as one technique, and using the ModuleRef class to retrieve a provider instance from the DI container as another.
We also describe resolving circular dependencies between modules.
warning Warning A circular dependency might also be caused when using "barrel files"/index.ts files to group imports. Barrel files should be omitted when it comes to module/provider classes. For example, barrel files should not be used when importing files within the same directory as the barrel file, i.e.
cats/cats.controllershould not import
catsto import the
cats/cats.servicefile. For more details please also see this github issue.
A forward reference allows Nest to reference classes which aren't yet defined using the
forwardRef() utility function. For example, if
CommonService depend on each other, both sides of the relationship can use
@Inject() and the
forwardRef() utility to resolve the circular dependency. Otherwise Nest won't instantiate them because all of the essential metadata won't be available. Here's an example:
info Hint The
forwardRef()function is imported from the
That covers one side of the relationship. Now let's do the same with
warning Warning The order of instantiation is indeterminate. Make sure your code does not depend on which constructor is called first.
An alternative to using
forwardRef() is to refactor your code and use the
ModuleRef class to retrieve a provider on one side of the (otherwise) circular relationship. Learn more about the
ModuleRef utility class here.
In order to resolve circular dependencies between modules, use the same
forwardRef() utility function on both sides of the modules association. For example: